As is known in the art, a pulse width modulator (PWM) circuit for switching a power switching device on and off may be provided by a comparator which compares an output signal from a control amplifier with a ramp waveform signal. The circuit may be configured such that an output load switch is on when the control amplifier output signal is greater than the ramp voltage and conversely, the output switch is off when the voltage level of the ramp waveform has a value greater than the output voltage of the control amplifier. Thus, the length of time the switch is on may be controlled by the output voltage of the control amplifier.
The ramp waveform typically completes one cycle between a minimum value and a maximum value within each period of the system's switching frequency. With this technique the amount of energy delivered to a load may be controlled by the length of time the switch is on during each period as compared to the total period of each switching cycle.
The ramp waveform may be provided by charging a capacitor with a current derived from the input supply voltage. This may be accomplished by coupling a resistor between the input voltage source and a first terminal of the capacitor. Thus, the resistor is coupled to the same input voltage source which is supplying energy to the load.
With this technique, an increase in the input voltage level provides a concomitant increase in the current charging the capacitor. Thus the rising voltage level of the ramp signal will increase to a relatively steep slope which results in the ramp signal voltage becoming greater than the voltage level of the control amplifier output signal in a relatively short period of time.
Similarly, as the input voltage is lowered, the slope of the ramp signal voltage decreases such that it takes a longer period of time for the voltage level of the ramp signal to increase past the control amplifier output voltage level. This results in a longer output pulse. Thus, when the input voltage is relatively large the output pulse-width is narrow and when the input voltage is relatively small the output pulse is wide with no action necessary from the control amplifier.
Energy is proportional to the product of the voltage amplitude and the time duration that the voltage is applied to the load. Thus, the above arrangement, generally referred to as a voltage feedforward circuit, may ideally deliver constant energy to the output regardless of the input voltage.
Such a circuit may be operated at a constant switching frequency by coupling a switch in parallel with the capacitor terminals and switching the switch via a fixed frequency oscillator. The switch should be switched for a duration of time long enough to discharge the capacitor. In the on state the switch provides a very low impedance and thus the capacitor rapidly discharges in an amount of time which is negligible with respect to the time of a full period.
In such circuits however, it is often desirable to provide a time interval during which the switch is assured to be off. This is generally referred to as a dead-time period. The dead-time is provided to allow time for the reset of magnetic components within the power supply. Thus, the range of the duty cycle modulation is limited to insure that there always exists a dead-time period.
To slow the discharge of the capacitor through the switch, a resistor may be coupled between a first terminal of the switch and a first terminal of the capacitor. The circuit may thus be arranged such that while the capacitor is discharging the output transistor may be turned off. Thus, by selecting resistors having predetermined values, a dead-time may be programmed into the circuit.
It is difficult, however, to provide a circuit which operates at a constant switching frequency and which has both voltage feedforward circuit characteristics and a duty cycle clamp. This is because the variable charging time of the capacitor tends to move the discharge time with respect to the total switching period and either switching frequency may vary, or at low input voltages, the capacitor may not be completely discharged by the end of the period. This results in a reduction in the dead-time period. Thus, it has been relatively difficult to provide a circuit operating at a constant switching frequency having both voltage feedforward characteristics and duty cycle clamp characteristics.